Recent genomic studies have revealed the highly polygenic nature of psychiatric disorders, including schizophrenia, bipolar disorder and major depressive disorder. Many of the individual genetic associations are shared across multiple disorders in a way that points to extensive biological pleiotropy and further challenges the biological validity of existing diagnostic approaches. Here we argue that the existence of risk alleles specific to a single diagnostic category is unlikely. We also highlight some of the important clinical repercussions of pleiotropy.